Dual-Bed Catalytic Distillation Tower And Method For Preparing Dimethyl Ether Using The Same

ABSTRACT

A dual-bed catalytic distillation tower has a catalytic column from the top down having an upper catalytic bed filled with low temperature dehydration catalysts and a lower catalytic bed filled with high temperature dehydration catalysts. When using the dual-bed catalytic distillation tower, the feeding may be fed to the tower from the top of the upper catalytic bed, between the upper and lower catalytic beds or the bottom of the lower catalytic bed for dehydration to obtain DME. The dual-bed catalytic distillation tower has the advantage of flexible set up depending on various feedings such as anhydrous or crude methanol and on different grade of DME to be obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a distillation tower used for preparingdimethyl ether (DME), especially a dual-bed catalytic distillationtower. The present invention also relates to a method for preparingdimethyl ether using the same.

2. Description of the Prior Arts

Dimethyl ether (DME) is prepared by using different raw materials suchas coal, nature gas, petroleum coke and biochar. The methanol is firstlyprepared from the raw materials, and the methanol is dehydrated toobtain DME. In the dehydration reaction, catalysts are needed to carryout the reaction. Solid-state acid catalysts such as zeolite, silicone,aluminum oxide, resin etc. (Spivey, J. J., 1991) or modified acid ionexchange resin (U.S. Pat. No. 6,740,783) are commonly used in thedehydration. Resin catalysts are low temperature catalysts with reactiontemperature ranging from 70 to 150° C. and almost 100% conversion ofmethanol can be obtained. However, the low temperature catalysts cannotbear high temperature. Other catalysts are high temperature catalystswith reaction temperature ranging from 200 to 350° C. When the reactiontemperature is getting higher, however, methanol and DME will be furtherdehydrated to C2-C4 olefin and results in the activity of catalystsdecay and DME yield reduce.

Further, to reduce the energy consumption during the distillationprocess, the catalysts are put in the distillation tower so the heatgenerated from the methanol dehydration reaction can be used fordistillation. The requirement for this application is the catalyticreaction and distillation can happen at the same tower pressure andtemperature range.

US 2007/0066855 A1 discloses a method for production of DME usingcatalytic distillation tower so the heat generated from dehydration canbe fully used and reduce the production cost. However, the towertemperature and pressure used for dehydration reaction cause methanoland water in the stripping column cannot be separated easily. FIG. 1shows the relative volatility of methanol and water at different towerpressure and can be used to calculate the difficulty for separatingmethanol and water at high tower pressure. FIG. 1 shows that therelative volatility of methanol and water reaches 1 (i.e. the azeotropiczone) when the mole fraction of liquid water equals to 0.825 at towerpressure of 18 bar. Azeotropism of methanol and water results inseparation difficult.

WO 2007/014534 provides high temperature dehydration in the catalyticdistillation tower by use of high temperature dehydration catalysts atreaction temperature from 160 to 180° C. and under tower pressure from1.8 to 2.3 bar. However, the energy consumption of the condenser andreboiler increases in order to drive the catalytic reaction under hightemperature.

U.S. Pat. No. 5,684,213 mentions high temperature dehydration reactionat reaction temperature from 350 to 400° C. and under 600 psi towerpressure. When dehydration is carried out at such high temperature,olefin side products increase and cause the activity of catalysts decaysand the DME yield decreases. Hydrogen is added to the catalyticdistillation tower to inhibit undesired side products generated.However, adding hydrogen increases the production cost and the reactioncomplexity.

The technique of using low temperature dehydration catalysts in thecatalytic distillation tower is also disclosed. For example, WO2007/014534 provides that dehydration reaction is carried at reactiontemperature from 130 to 158° C. and under tower pressure from 10 to 18bar by using low temperature acid ion exchange catalyst. However, thelow temperature catalysts only can be used at temperature lower than140° C. When the reaction is carried out at low temperature andpressure, the dew point of high purity DME on the top of the tower willdecrease. For example, the dew point of DME becomes 40.3° C. when thetower pressure is low to 9 bar and therefore using 20° C. industrialwater to condense DME will be impossible. When the tower pressure isadjusted to 12 bar, the temperature at the catalytic column will behigher than 150° C., and the low temperature catalysts will be damaged.

All the above-mentioned methods use single-bed catalytic system in thedistillation tower, i.e. either high temperature or low temperaturedehydration reaction is carried out. In the conventional methods, thedistillation tower cannot be easily adjusted depending on the grade ofmethanol feeding and DME to be obtained because none of the conventionalmethod can provide suitable and complementary tower temperature andpressure conditions.

SUMMARY OF THE INVENTION

Therefore, the purpose of the present invention is to provide acatalytic distillation method for preparing dimethyl ether (DME) inwhich the tower temperature and pressure can be proper adjusteddepending on the relation among DME, methanol and water in thedistillation and the dehydration catalysts selected.

Specifically, the present invention relates to a dual-bed catalyticdistillation tower and the method for preparing DME using the same.

The present invention provides a dual-bed catalytic distillation towercomprising a catalytic column. The catalytic column from the top downcomprises an upper catalytic bed and a lower catalytic bed. The uppercatalytic bed is filled with low temperature dehydration catalysts andthe lower catalytic bed is filled with high temperature dehydrationcatalysts. At least one feed port is set on the top of the uppercatalytic bed, between the upper catalytic bed and the lower catalyticbed and on the bottom of the lower catalytic bed.

Preferably, the low temperature dehydration catalyst is Amberlyst® 15acid ion exchange resin or Amberlyst® 35 acid ion exchange resin.

Preferably, the high temperature dehydration catalyst is fluoridizedtransition metal oxide, sulfate transition metal oxide, βzeolite andHZSM-5. All high temperature dehydration catalysts are covered by Teflonto enhance hydrophobic property of the catalysts.

The present invention also provides a method for preparing dimethylether by using aforesaid dual-bed catalytic distillation tower whereinthe catalytic column is used for dehydration of methanol. The feedingstream containing methanol is fed from the feed port located on the topof the upper catalytic bed, between the upper catalytic bed and thelower catalytic bed or on the bottom of the lower catalytic bed at thetower pressure from 6 to 30 bar for dehydration to obtain DME.

The temperature of the catalytic distillation tower of the presentinvention is at the range from 60 to 250° C. When the tower temperatureis at the range from 60 to 180° C., the low temperature dehydrationcatalysts are used for dehydration. When the tower temperature is at therange from 110 to 250° C., the high temperature dehydration catalystsare used for dehydration.

Preferably, the tower pressure of the catalytic distillation tower isranging from 8 to 14 bar.

Preferably, the catalytic distillation tower further comprises at leastone flash zone on the on the top of the upper catalytic bed, between theupper catalytic bed and the lower catalytic bed and on the bottom of thelower catalytic bed is reserved for heat exchange and reflux.

When anhydrous methanol is used as the feeding, it is fed to the towerfrom the feed port located on the top of the upper catalytic bed.

When syngas is used as the feeding, it is firstly converted to a mixturecontaining DME, methanol and water, and the mixture is fed to the towerfrom the feed port located on the top of the upper catalytic bed orbetween the upper catalytic bed and the lower catalytic bed.

When crude methanol is used as the feeding, it is feed to the tower fromthe feed port located on the bottom of the lower catalytic bed orbetween the upper catalytic bed and the lower catalytic bed.

Comparing to the conventional single-bed catalytic distillation tower,the dual-bed catalytic distillation tower of the present invention hasadvantages of flexible set up depending on various feedings such asanhydrous or crude methanol and on different grade of DME to beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram shown the changes of relative volatility of methanoland water at different tower pressure;

FIG. 2 is a liquid-gas phase diagram shows the equilibrium relationbetween methanol and DME as well as methanol and water at differenttemperature under 11 bar;

FIG. 3 is a schematic diagram shows the dual-bed catalytic distillationof the present invention;

FIG. 4 is a schematic diagram shows DME preparation methods using thedual-bed catalytic distillation tower of the present invention;

FIG. 5 is a diagram shown the tower temperature distribution in thedistillation tower of the present invention with anhydrous methanolfeeding;

FIG. 6 is a diagram shown the tower temperature distribution in thedistillation tower of the present invention with crude methanol feeding;and

FIG. 7 is a diagram shown the tower temperature distribution in thedistillation tower of the present invention with mixture feeding of DME,methanol and water.

DETAILED DESCRIPTION OF THE INVENTION

The above and other technical features and advantages of the presentinvention will be described in greater detail with reference to thedrawings.

FIG. 2 is a liquid-gas phase diagram shows the equilibrium relationbetween two substances at different temperature under 11 bar. The rightpart of FIG. 2 shows the equilibrium relation between methanol anddimethyl ether (DME). As shown in FIG. 2, when the temperature is closeto the saturated liquid temperature of methanol, i.e. 140.5° C., theconcentration of methanol increases from the top to bottom of thedistillation tower and the concentration of liquid phase methanol ishigher than the concentration of gas phase methanol at the same positionof the tower. Therefore, the low temperature dehydration catalyst can beused for dehydration when it is preformed under the conditions shown inZ1 zone in FIG. 2. The left part of FIG. 2 shows the equilibriumrelation between methanol and water. When the temperature is close tothe saturated liquid temperature of methanol, the concentration ofmethanol increases from bottom to top of the distillation tower and theconcentration of gas phase methanol is higher than the concentration ofliquid phase methanol at the same position of the tower. Therefore, thehigh temperature dehydration catalyst can be used for dehydration whenit is preformed under the conditions shown in Z2 zone in FIG. 2. Thesaturated liquid temperature of methanol varies with tower pressure.When the tower pressure is lower than 11 bar, the saturated liquidtemperature of methanol becomes lower than 140.5° C. On the contrary,the saturated liquid temperature of methanol becomes higher than 140.5°C. when the tower pressure is higher than 11 bar.

Based on the substance properties as described above, the presentinvention provides a catalytic distillation tower where differentcatalysts can be placed in different temperature zones to carry outdehydration of methanol efficiently and prevent the catalysts from beingdamaged due to high temperature. As shown in FIG. 3, the catalyticcolumn in the catalytic distillation tower of the present inventioncomprises an upper catalytic bed 7 and a lower catalytic bed 11. Threeflash zones 8, 9, 10 are respectively located on the top of the uppercatalytic bed 7, between the upper catalytic bed 7 and the lowercatalytic bed 11 and on the bottom of the lower catalytic bed 11. Threefeed ports 1, 2, 3 are respectively located on the flash zones 8, 9, 10.A heat exchanger and/or reflux condenser 12 is located between uppercatalytic bed 7 and the lower catalytic bed 11.

Generally, the temperature of the catalytic distillation tower of thepresent invention is from 60° C. to 250° C. When the tower temperatureis from 60° C. to 180° C., the low temperature dehydration catalyst isused in Z1 shown in FIG. 2, i.e. high liquid phase methanolconcentration zone. When the tower temperature is from 110° C. to 250°C., the high temperature dehydration catalyst is used in Z2 shown inFIG. 2, i.e. high gas phase methanol concentration zone. The criticaltemperature range to decide using high temperature dehydration catalystor low temperature dehydration catalyst is from 110° C. to 180° C.

Preferably, the low temperature dehydration catalyst is used in theupper catalytic bed 7. For example, but not limited to, Amberlyst® 15acid ion exchange resin is used at tower temperature from 85 to 110° C.and Amberlyst® 35 acid ion exchange resin is used at tower temperaturefrom 110 to 135° C. The high temperature dehydration catalyst is used inthe lower catalytic bed 11. For example, fluoridized transition metaloxide such as F-alumina, sulfate transition metal oxide such as sulfatezirconium dioxide (SO₄ ²⁻/ZrO₂), βzeolite and HZSM-5 are suitable hightemperature dehydration catalyst. The high temperature dehydrationcatalyst is covered by Teflon to enhance hydrophobic property of thecatalyst and prevent the mass transfer resistance caused by the liquidmolecular on the surface of the catalyst.

The tower pressure varies depending on the environment. The pressure isusually at the range from 6 bar to 30 bar, preferably from 8 bar to 14bar. If the distillation tower is used at high latitudes, consideringusing 20° C. industrial water to condense the DME with dew point 25.4°C., the tower pressure can be low to 6 bar. When the tower pressure isup to 18 bar, the relative volatility of methanol and water should beconsidered.

The feeding can be feed to the distillation tower from different feedports 1, 2 and 3. When anhydrous methanol is used as the feeding, it isusually dehydrated in liquid phase so it is feed to the tower from feedport 1 located on the top of the upper catalytic bed 7. When syngas isused as the feeding, it firstly converts to a mixture containingdimethyl either, methanol and water, and the mixture is usually feed tothe tower from feed port 1 located on the top of the upper catalytic bed7 or feed port 2 located between the upper catalytic bed 7 and the lowercatalytic bed 11. When crude methanol is used as the feeding, it isfirstly heated to saturated vapor for further reaction so it is usuallyfeed to the tower from feed port 3 located on the bottom of the lowercatalytic bed 11 or feed port 2 between the upper catalytic bed 7 andthe lower catalytic bed 11. The heat exchanger and/or reflux condenser12 located between the upper catalytic bed 7 and the lower catalytic bed11 is used to control the temperatures of the two catalytic beds andsimultaneously protect the low temperature dehydration catalysts in theupper catalytic bed 7 from being damaged due to high temperature.

As shown in FIG. 3, the distillation tower further contains arectification column 13 on the top of the catalytic column and astripper column 14 under the catalytic column. After reaction in thedistillation tower, the mixture of methanol and carbon dioxide isdischarged from a condenser outlet 4 and DME is discharged and collectedfrom a condenser outlet 5. Unreacted feeding stream flows into thereboiler on the bottom of the tower for separation. After the reactionin the reboiler, water goes out from a reboiler outlet 6 and othersubstances in the feeding stream flow back to the tower for nextreaction.

FIG. 4 shows the application of the dual-bed catalytic distillationtower in the preparation of DME. FIG. 4( a) shows how the dual-bedcatalytic distillation tower works in one-step preparation of DME. Afterthe one-step reaction, mixture stream 23 containing DME, methanol, waterand carbon dioxide may be fed into the dual-bed catalytic distillationtower from different feed ports 1, 2 and 3 for dehydration to obtainDME. FIG. 4( b) shows how the dual-bed catalytic distillation towerworks in two-step preparation of DME. After two-step reaction, the crudemethanol stream 22 and anhydrous methanol stream 21 may be fed into thedual-bed catalytic distillation tower from different feed ports 1, 2 and3 for dehydration to obtain DME.

EXAMPLES

In the following examples, low temperature dehydration rate (An et al.,2004), thermodynamic NRTL-RK equation, the thermodynamic data for hightemperature dehydration reaction (Lin et al., 1981; Hayashi, 1982) areused for theoretical calculation of the DME catalytic distillation towerby the simulation software “Aspen Plus” to illustrate the feasibility ofthe present invention.

The catalytic distillation tower from the top down contains an overheadcondenser, rectification column, catalytic column, stripping column anda reboiler at tower bottom.

The conditions for the conventional single-bed catalytic distillationtower are set as follows. The theoretical plate numbers of therectification column and the stripping column are respectively set to 7(including part or all of the condenser and reboiler). The feedcontaining methanol mixture is fed to the distillation tower at 30° C.,and performs heat exchange with the hot water comes from the bottom ofthe distillation tower. The feed temperature is about 40° C. after theheat exchange step. The amount of catalyst and the tower height areadjusted, i.e. changing the plate number of the catalytic column and thetolerated flow rate for liquid in the catalytic column (24 inchheight/plate) to be 24.4 m/s. Afterwards, the reflux ratio and D/F areadjusted so the concentration of DME and water produced from thedistillation tower are respectively 99.9 wt %.

In the dual-bed catalytic distillation tower, the catalytic columnincludes an upper catalytic bed filled with low temperature dehydrationcatalysts and a lower catalytic bed filled with high temperaturedehydration catalysts. The position in the tower having temperature at135° C. is set as the demarcation point for upper and lower catalyticbeds. The plate number of the catalytic column, the reflux ratio and D/Fare adjusted so the concentration of DME and water produced from thedistillation tower are respectively 99.9 wt %. When the fuel grade DME(93 wt % of DME and 7 wt % of methanol) is desired under the samedistillation design, the amount of catalysts will be too much. In thissituation, the feeding increases so the conversion of methanol decreasesand the fuel grade DME is obtained from the top of the tower and theconcentration of the water obtained from the bottom of the distillationtower is still 99.9 wt %.

The conditions of the present dual-bed catalytic distillation tower areassumed as below. The unit volume of the upper catalytic bed equals to0.6 unit volume of the theoretical plate and the unit volume of thelower catalytic bed equals to 0.85 unit volume of the theoretical plate(referring to U.S. Pat. No. 6,045,762 and US 2007/U.S. Pat. No.0,095,646). The dehydration rate of the hydrophobic and strong acidiccatalysts is five times more than the rate disclosed in the literatureLin et al., 1981.

COMPARATIVE EXAMPLE Anhydrous Methanol as Feeding (Prior Art)

The simulation for the production of DME in US 2007/U.S. Pat. No.0,066,855 is indicated below.

Tower pressure 12 bar Pressure drop between each plate 0.7 bar Refluxratio  2 D/F weight ratio  0.71773 Theoretical plates 30 Feeding plateAt plate number 8 Catalyst column From plate number 10 to 18 Catalystamount 140 m³ Methanol fed 5013 tpd (99.88 weight %) DME produced 3596tpd (92.35 weight %) Temperature on the top of the tower 56.1° C.Temperature on the bottom of the tower 18.9° C.

The practice result shows that the DME produced is 3598 tpd (99.61weight %) and the temperatures on the top and bottom of the tower isrespectively 52° C. and 190° C. Although the tower temperaturedistribution does not disclosed in the prior art, the temperature isassumed having 4° C. inaccuracy compared to the simulated towertemperature.

The practice result shows that the temperature at the bottom two plate,i.e. plate number 17 and 18, is respectively 151.5° C. and 156.4° C.which exceed the limit of temperature resistance of acid ion exchangeresin catalysts such as Amberlyst® 35 having temperature resistance at140° C. Therefore, the low temperature dehydration catalysts cannot beused in the conventional distillation tower under certain tower pressuresuch as at the pressure ranging from 8 to 12 bar.

Example 1 Anhydrous Methanol as Feeding

The simulation for the production of DME in the dual-bed distillationtower of the present invention and in the conventional single-beddistillation tower is indicated below.

Tower pressure   11 bar Pressure drop 0.025 bar Initial reflux ratio 4D/F molar ratio 0.4997 Feeding plate At plate number 7

The practice results show in Table 1, Table 2 and FIG. 5.

TABLE 1 Stream composition during the distillation process CatalyticColumn Single-bed* Dual-bed Dual-bed DME grade Chemical Chemical FuelStream*** 1 5 6 1 5 6 2 5 6 Water, kg/hr 6 0 1689 6 0 1689 7 0.1 1791Methanol, kg/hr 5994 4.3 1.7 5994 4.3 1.7 6693 343 1.8 DME, kg/hr 0 43050 0 4305 0 0 4564 0 Total, kg/hr 6000 4309 1691 6000 4309 1691 6700 49071793 *Conventional method, the low temperature dehydration catalyst doesnot work at high temperature. ***Stream composition at the feed port 1and 2, condenser output 5 and reboiler output 6 shown in FIG. 3.

TABLE 2 The practice results for the preparation of DME using single-bedand dual-bed distillation tower Catalytic Column Single-bed* Dual-bedDual-bed DME grade Chemical Chemical Fuel D/F, mole 0.4993 0.4993 0.5246Reflux ratio 1.728 1.768 1.106 Feeding plate 7 7 7 number Plate numberCondenser 1 1 1 Rectification 2-7 2-7 2-7 column Upper catalytic bed 8-20  8-16  8-16 Lower catalytic bed N/A 17-24 17-24 Stripping column21-26 25-30 25-30 Reboiler 27 31 31 Low temperature 19.7 13.7 13.7catalyst, m³ High temperature N/A 17.2 17.2 catalyst, m³ Towertemperature, Rectification 48.3-108  48.3-108   52-110 column Uppercatalytic bed 108-178 108-134 110-135 Lower catalytic bed N/A 139-177139-177 Stripping column 182-187 183-187 183-187 Pressure at tower 11 1111 top, bar *Conventional method, the low temperature dehydrationcatalyst does not work at high temperature.

As shown in Table 2, 4 more plates are needed in the dual-beddistillation tower compared to the single-bed distillation tower inorder to increase methanol conversion rate. This is because the reactionrate of the high temperature dehydration catalyst is lower than the rateof the low temperature dehydration catalyst so the amount of hightemperature dehydration catalyst is more than the amount of lowtemperature dehydration catalyst. In the present invention, hightemperature dehydration catalysts (from plate 17 to 24) are used toreplace part of the low temperature dehydration catalysts (from platenumber 17 to 20) in the conventional distillation tower so thetemperature increases moderately at the lower catalytic bed. Therefore,the temperature at the border between the upper catalytic bed and thelower catalytic bed would not be too high and damage the low temperaturedehydration catalysts in the upper catalytic bed.

Example 2 Crude Methanol as the Feeding

The feeding comes from the process for preparing methanol from syngas(with reference to U.S. Pat. No. 5,750,799). The produced methanolcontains 10-20 mole % of water. The produced methanol is fed to thecatalytic distillation tower of the present invention directly fordehydration without being purified. The simulation for the production ofDME in the dual-bed distillation tower of the present invention and inthe conventional single-bed distillation tower is indicated below.

Tower pressure   11 bar Pressure drop 0.025 bar Initial reflux ratio 4D/F molar ratio 0.4997

The results of using dual-bed distillation tower of the presentinvention and using conventional single-bed distillation tower are shownin Table 3, Table 4 and FIG. 6.

TABLE 3 Stream composition during the distillation process CatalyticColumn Single-bed* Dual-bed Dual-bed Single-bed** DME grade ChemicalChemical Chemical Chemical Feeding plate number**** 7 7 9 16 Stream***feeding 5 6 5 6 5 6 5 6 water, kg/hr 666 0 2349 0 2349 0.08 2336 0 2349methanol, kg/hr 5994 4.3 2.4 4.3 2.4 28.3 26.6 4.3 2.4 DME, kg/hr 0 43040 4304 0 4270 0 4304 0 Total, kg/hr 6660 4309 2351 4309 2351 4298 23624309 2351 *Conventional method, the low temperature dehydration catalystdoes not work at high temperature. **Conventional method of using hightemperature catalysts. ***Stream composition at condenser output 5 andreboiler output 6 shown in FIG. 3. ****Feeing at plate number 7 meansfeeding from feed port 1, feeding at plate number 9 means feeding fromfeed port 2, feeding at plate number 16 means feeding from feed port 3.

TABLE 4 The practice results for the preparation of DME using single-bedand dual-bed distillation tower with crude methanol Catalytic ColumnSingle-bed* Dual-bed Dual-bed Single-bed** DME grade Chemical ChemicalChemical Chemical Feeding plate 7 7 9 16 number D/F, mole 0.4176 0.41760.4176 0.4176 Reflux ratio 1.228 1.28 2.86 13.1 Plate number Condenser 11 1 1 Rectification 2-7 2-7 2-6 2-7 Column Upper catalytic bed  8-12 8-97-8 N/A Lower catalytic bed N/A 10-17 10-17  8-15 Stripping column 13-1818-23 18-23 16-21 Reboiler 19 24 24 22 Low temperature 11.6 4.66 4.66N/A catalyst, m³ High temperature N/A 26.4 26.4 22.6 catalyst, m³ Towertemperature, Rectification 48.3-100  48.3-101  48.6-122  48.3-154 column Upper catalytic bed 120-170 120-131 125-128 N/A Lower catalyticbed N/A 136-171 136-160 164-174 Stripping column 177-186 178-186 168-186172-186 Pressure at tower 11 11 11 11 top, bar *Conventional method, thelow temperature dehydration catalyst does not work at high temperature.**Conventional method, use high temperature dehydration catalyst.

When the methanol is fed from feed port 1, the liquid flow amount ismore than the amount in Example 1 because the methanol fed containingwater. Therefore, the tower diameter in this example is larger than thetower diameter in Example 1. The theoretical calculated result of thetemperature distribution in the tower is similar to the towertemperature distribution shown in FIG. 5. The temperature of the lowercatalytic bed in the dual-bed distillation system is lower than thetemperature in the single-bed distillation system at the same position.Accordingly, the low temperature dehydration catalyst is betterprotected in dual-bed distillation system.

When the upper catalytic bed is moved a plate upper and methanol is fedfrom feed port 2, the reflux rate is raised to increase the conversionrate of methanol. The concentrations of DME on the tower top and wateron the tower bottom respectively decrease to 99.3 wt % and 98.9 wt %.

When methanol is fed from feed port 3, only high temperature dehydrationcatalysts can be used in the single-bed distillation system because lowtemperature dehydration catalysts will be damaged when the columntemperature is higher than 160° C.

Example 3 Mixture of DME, Methanol and Water as the Feeding

The feeding comes from the process for preparing methanol from syngas(with reference to U.S. Pat. No. 5,908,963). After separation, theconcentration of carbon dioxide, DME, methanol and water in the productfrom the reactor are respectively 2.8 wt %, 49.7 wt %, 31.0 wt % and16.5 wt %. The mixture can be fed from the top catalytic column orbetween the upper and lower catalytic beds. The reflux ratios arecalculated to be 0.378 and 1.22. The practice results are shown in Table5, Table 6 and FIG. 7.

TABLE 5 Stream composition during the distillation process CatalyticColumn Single-bed* Dual-bed Dual-bed DME grade Chemical ChemicalChemical Feeding plate number**** 7 7 9 Stream*** 1 5 6 1 5 6 2 5 6Water, kg/hr 3190 0 4870 3190 0 4870 3190 0 4864 Methanol, kg/hr 599413.6 4.9 5994 13.7 4.9 5994 25 15 DME, kg/hr 9610 13905 0 9610 13905 09610 13890 0 Total, kg/hr 18794 13919 4875 18794 13919 4875 18794 139154879 *Conventional method, the low temperature dehydration catalyst doesnot work at high temperature. ***Stream composition at the feed port 1and 2, condenser output 5 and reboiler output 6 shown in FIG. 3.****Feeing at plate number 7 means feeding from feed port 1, feeding atplate number 9 means feeding from feed port 2.

TABLE 6 The practice results for the preparation of DME using single-bedand dual-bed distillation tower Catalytic column Single-bed* Dual-bedDual-bed DME grade Chemical Chemical Chemical Feeding plate 7 7 9 numberD/F, mole 0.5278 0.5278 0.5278 Reflux ratio 0.354 0.378 1.22 Platenumber Condenser 1 1 1 Rectification 2-7 2-7 2-6 Column Upper catalyticbed  8-12 8-9 7-8 Lower catalytic bed N/A 10-14 10-14 Stripping column13-18 15-20 15-20 Reboiler 19 21 21 Low temperature 16.4 6.6 6.6catalyst, m³ High temperature N/A 23.3 23.3 catalyst, m³ Towertemperature, Rectification 48.3-64   48.3-66   48.3-96   column Uppercatalytic bed  89-173  94-125 101-104 Lower catalytic bed N/A 140-172134-167 Stripping column 179-186 179-186 174-186 Pressure at tower 11 1111 top, bar *Conventional method, the low temperature dehydrationcatalyst does not work at high temperature.

FIG. 7 shows the temperature changes in the catalytic column. Feedingthe mixture from the feed port 2 between the upper and lower catalyticbeds alleviates rapid temperature changes in the catalytic column andsimultaneously protect the catalysts in the low temperature catalyticbed from being damaged. As shown in FIG. 3, heat exchange/reflux may becarried out at the flash zone 9 to control the temperature at thecatalytic column and protect the low temperature dehydration catalystfrom being damaged.

Example 4 Mixture of DME, Methanol, Carbon Dioxide and Water as theFeeding

The feeding comes from the process for preparing methanol from syngas(with reference to U.S. Pat. No. 5,908,963). After separation, theconcentration of carbon dioxide, DME, methanol and water in the productfrom the reactor are respectively 2.8 wt %, 49.7 wt %, 31.0 wt % and16.5 wt %. The carbon dioxide can not be liquidized because of thelimitation of the tower pressure. Thus, an overhead condenser isequipped on the top of the distillation tower. The results are shown inTable 7. The DME obtained contains 1.36 wt % of carbon dioxide so itwill flow back to the distillation tower for purify. The gas phasecarbon dioxide and DME are respectively 25 wt % and 75 wt %. They alsoneed to be re-purified in the distillation tower.

TABLE 7 The results for the preparation using dual-bed distillationtower Catalytic Dual-bed column Feeding plate 7 number**** Stream*** 1 45 6 Water 3101 0 0.6 4733 Methanol 5826 0.1 12.7 4.7 Dimethyl ether 93411038 12479 0 Carbon dioxide 526 354 172 0 Total, kg/hr 18794 1392 126644738 Results D/F, mole 0.538 Gas/tower top 0.1 distillate ratio Refluxratio 0.343 Plate number Condenser 1 Rectification 2-7 Column Uppercatalytic 8-9 bed Lower catalytic 10-13 bed Stripping 14-19 columnReboiler 20 Low 6.9 temperature catalyst, m³ High 19.4 temperaturecatalyst, m³ Tower temperature, Rectification 41-68 column Uppercatalytic  80-112 bed Lower catalytic 143-185 bed Stripping 190-193column Pressure at 13 tower top, bar ***Stream composition at the feedport 1 and 2, condenser output 5 and reboiler output 6 shown in FIG. 3.****Feeing at plate number 7 means feeding from feed port 1.

What stated above is only preferred embodiments of the presentinvention, which is illustrative only and not restrictive. Many changes,modifications, or the equivalents may be made by those skilled in theart without departing from the spirits and scope of the presentinvention as defined by the claims, but will fall within the scope ofprotection of the present invention.

1. A dual-bed catalytic distillation tower comprising a catalytic columnfrom the top down having an upper catalytic bed and a lower catalyticbed, at least one feed port set on the top of the upper catalytic bed,between the upper and lower catalytic beds and under the lower catalyticbed; wherein the upper catalytic bed is filled with low temperaturedehydration catalysts and the lower catalytic bed is filled with hightemperature dehydration catalysts.
 2. The dual-bed catalyticdistillation tower according to claim 1, wherein the low temperaturedehydration catalyst is Amberlyst® 15 acid ion exchange resin orAmberlyst® 35 acid ion exchange resin.
 3. The dual-bed catalyticdistillation tower according to claim 1, wherein the high temperaturedehydration catalyst is fluoridized transition metal oxide, sulfatetransition metal oxide, β zeolite and HZSM-5.
 4. The dual-bed catalyticdistillation tower according to claim 1, wherein the high temperaturedehydration catalysts are covered by Teflon.
 5. A method for preparingdimethyl ether (DME) by using the dual-bed catalytic distillation toweraccording to claim 1, wherein a feeding containing methanol is fed tothe distillation tower from the top of the upper catalytic bed, betweenthe upper and lower catalytic beds or the bottom the lower catalytic bedat tower pressure from 6 to 30 bar for dehydration to obtain DME.
 6. Themethod for preparing DME according to claim 5, wherein the towerpressure is from 8 to 14 bar.
 7. The method for preparing DME accordingto claim 5, wherein the tower temperature is at the range from 60 to250° C.; the low temperature dehydration catalysts are used fordehydration when the tower temperature is from 60 to 180° C.; the hightemperature dehydration catalysts are used for dehydration when thetower temperature is from 110 to 250° C.
 8. The method for preparing DMEaccording to claim 5, wherein at least one flash zone on the on the topof the upper catalytic bed, between the upper catalytic bed and thelower catalytic bed and on the bottom of the lower catalytic bed isreserved for heat exchange and reflux.
 9. The method for preparing DMEaccording to claim 5, wherein the feeding is anhydrous methanol and fedfrom the top of the upper catalytic bed.
 10. The method for preparingDME according to claim 5, wherein the feeding is the mixture of DME,methanol and water obtained from syngas and fed between the upper andlower catalytic beds.
 11. The method for preparing DME according toclaim 5, wherein the feeding is crude methanol and fed from the bottomof the lower catalytic bed or between the upper and lower catalyticbeds.